Internal combustion engine cylinder bores and cylinder liner bores are conventionally heat treated by electrical induction devices which travel longitudinally therethrough in order to surface harden that portion of the bore that experiences piston wear. The circumferential, red hot band resulting from the electrical heating field, is normally followed as it moves through the bore by an umbrella of cooling fluid that impinges on the band axially away from the heating field. While this technique is being used with relative success by the industry, it requires a relatively expensive manufacturing apparatus, and despite careful control of operational parameters, non-uniform depths of heat treatment and hardness are obtained. For example, one of the problems inherent with induction heating systems is that surface or subsurface irregularities tend to interrupt the electric field and thereby cause localized concentrations of heat. Such heat concentrations result in a variable microstructure of the bore, which leads to cracking problems in the workpiece. Further, high energy levels, for example, in the range of 150 to 400 kilowatts, tend to direct an excessive amount of heat into the workpiece thereby causing distortion thereof and often times an adverse affect on the metallurgical structure of nearby areas. Such latter problem is particularly acute on engine blocks with very closely spaced individual bores. In addition, induction heat treating is limited to elongated cylindrical areas and is relatively impractical for heat treating narrow or interrupted bands.
Reference is made to U.S. Pat. No. 3,772,496 to Harendza-Harinxma which discloses a relatively involved method and apparatus for removing a copper layer from the inner wall of a tube. Such patent teaches use of a concentrated beam of laser energy which is reflected from a mirror upon the inner wall to vaporize a helical path through the copper layer. While this patent is of some interest, it does not relate to heat treating a bore of a workpiece or solving the particular problems inherent therewith to which the present invention is particularly directed.